1. Field of the Invention
The present invention relates to a stack type flip-chip package. More particularly, the present invention relates to a stack type flip-chip package having a redistribution circuit on the back surface of a chip to serve as an electrical bridge for an upper chip.
2. Description of Related Art
To form a flip-chip package, a silicon chip having an active surface with many bonding pads thereon is provided. Each bonding pad is formed with a bump thereon, so that the chip can be flipped over and attached to the contact points on a package carrier. In general, a flip-chip package occupies a small area and has a high packaging density. Moreover, direct electrical connection through bumps reduces signal transmission path. Therefore, flip-chip designs are employed in chip scale packages (CSP), ball grid array (BGA) packages and multi-chip module (MCM). Note that a number of chips each having a different function can be enclosed within a single package to form a multi-chip module, which not only reduces the package volume and area, but also shortens the transmission path between chips. Consequently, the overall performance is improved.
To form a conventional multi-chip package, both flip-chip technique and chip stacking designs are employed so that two or more chips each having a different function are enclosed within a single package. As before, combining the flip-chip technique with a stacking design not only reduces the package area and increases the packing density, but also shortens the signal transmission length and improves the overall performance.
FIG. 1 is a schematic cross-sectional view of a conventional stack type flip-chip package. As shown in FIG. 1, the stack type flip-chip package 10 uses both the flip-chip technique and a stacking design such that an upper chip 14 and a lower chip 16 are packaged inside a substrate board 12. The upper chip 14 has a redistribution line 18. The redistribution line 18 not only provides a bonding pad 20 for connecting with a contact pad 24 on the substrate board 12 through a bump 22, but also provides another bonding pad 26 for connecting with a bump 28 on the lower chip 16. In addition, the bonding pad 26 also fans out to yet another bonding pad 30. The bonding pad 30 connects with a contact pad 34 on the substrate board 12 through a bump 32 so that the lower chip 16 and the substrate board 12 are electrically connected. To accommodate the lower chip 16, the substrate board 12 has an opening (or cavity) therein. The package also requires injection of some underfill material 38. The underfill material 38 fills up the space between the substrate board 12, the upper chip 14 and the lower chip 16 to protect the bumps, bonding pads and contact pads against shock and vibrations.
The aforementioned stack type flip-chip package design has the following drawbacks. The upper chip must provide sufficient number of bonding pads for connecting with the bumps on the lower chip. Hence, the usable area on the upper chip is greatly reduced and the bonding pad arrangement on the upper chip is severely limited. Moreover, the gaps between the substrate board and the upper and lower chip on one hand and between the upper and lower chip on the other hand are small and unevenly distributed. Thus, filling up all the space within the package with underfill material is difficult.